Hard mask films are commonly used as sacrificial layers during lithographic patterning, e.g., during trench and/or via formation in a Damascene process. In Damascene processing, a hard mask film is typically deposited onto a layer of dielectric that needs to be patterned. A layer of photoresist is deposited over the hard mask film (with an optional antireflective layer deposited between the hard mask and the photoresist), and the photoresist is patterned as desired. After the photoresist is developed, the exposed hard mask film below the pattern is removed, and the exposed dielectric is etched such that recessed features of required dimensions are formed. The remaining hard mask serves to protect those portions of dielectric that need to be preserved during the etching process. Therefore, the hard mask material should have a good etch selectivity relative to the dielectric. Reactive ion etching (RIE) which uses halogen-based plasma chemistry is typically employed for dielectric etching.
The etched recessed features are then filled with a conductive material, such as copper, forming the conductive paths of an integrated circuit. Typically, after the recessed features are filled, the hard mask material is completely removed from the partially fabricated semiconductor substrate.
Hard mask layers are becoming more common in both front-end-of-line (FEOL) back-end-of-line (BEOL) patterning schemes. For example, hard mask films are often used as an aid in critical patterning applications for BEOL inter-level dielectric (ILD) materials. These hard mask films should have a high etch selectivity with respect to the ILD material, be compatible with basic lithography processes and be capable of removal without damaging the underlying ILD layer. Currently, TiN is often used as a hard mask in BEOL low-k dielectric applications because of its very high etch selectivity relative to the low-k dielectric that facilitates its eventual removal. However, there are a number of integration issues associated with such a metal-based hard masks: (a) polymer formed to protect the sidewall of low-k ILD during the etch process reacts with metal-based hard mask to form a metal polymer residue that causes defect issues; (b) high compressive stress in the TiN coupled with weak mechanical properties of the low-k dielectric can lead to a buckling phenomenon, and (c) the need for a different etch platform for hard mask compared to underlayers, which can further increase cost.